This invention pertains to a method of extracting nickel, cobalt and other metals, including the platinum and palladium metal families, from soil by cultivation of the soil with hyperaccumulating plants that concentrate these metals in above-round portions of the plants, which can be harvested, dried and smeited to recover the metal (metal phytomining).
It has long been known that certain types of soil and geological materials, including serpentine, lateritic serpentine, ultramafic and meteor-impacted soils may be rich in nickel or cobalt, and are sites for mining of these metals. The cost of conventional mining for these metals remains high, and the level of metals required in geological materials to which current technology may be usefully applied are much higher than most serpentine, lateritic serpentine, ultramafic and meteor-derived soils.
This application is related to U.S. patent application Ser. No. 08/470,440, allowed, and its corresponding PCT application. In this earlier application, recovery of Ni, Co and related metals from soil is described through culturing Alyssum plants on Ni-enriched soil. The specific soil conditions described in that application include reducing calcium as far as possible, in accordance with conventional teachings regarding the inverse relationship between calcium concentration and nickel hyperaccumulation. Additionally, the application limits calcium concentrations to a value such that the exchangeable Ca/Mg ratio is below 0.20.
U.S. Pat. No. 5,364,451. Raskin et al., is directed to a method of removing metals from metal-rich soil by growing genetically altered plants of the family Brassicaceae in these soils, so as to remediate polluted soils at a reduced cost. Suitable parents for the mutants that are the subject of the Raskin patent include B. juncea. While the patent generally describes a large number of metals that may be recovered, specific artificial examples are directed to recovery of chromium and lead. The entire disclosure of U.S. Pat. No. 5,364,451 is incorporated herein by reference.
A review of the examples of this reference, and application of the technology proposed, illustrates continuing problems posed in rededication of metal-rich soil, and recovery of the metals therefrom. In particular, the examples set forth reflect artificial culture in sand media with intermittent feeding with phosphate to permit plants to grow without severe yield reduction and without severe lead toxicity. The patent also relies on genetic mutations that are produced by random xe2x80x9cmutagenesisxe2x80x9d, that is, the creation of a library of mutants or potential mutants from a starting parent by indiscriminate application of a mutagen, coupled with screening the offspring to define acceptable hyperaccumulators. While promising, the Raskin patent offers little basis for an opportunity to proceed directly with soil rededication through plant growth or culturing. Additionally, the patent offers little realistic opportunity for recovery of the metal itself, indicating only that under circumstances (not identified) the metal can actually be reclaimed.
One of the most widely found, and technologically important metals is nickel. Nickel is a natural constituent in all soils, being particularly high in concentration in serpentine, lateritic serpentine, ultramafic and meteor-derived soils. Cobalt, which has chemical and geological characteristics very similar to nickel, can similarly be found in these soils, and is another valuable metal. Other metals that are also subjects for phytomining within the scope of the invention, including those of the platinum and palladium families, including palladium, rhodium, ruthenium, platinum, iridium, osmium and rhenium which commonly co-occur with Ni and Co. Cultivation of plants which are hyperaccumulators of these metals, in metal-rich soils, or xe2x80x9cphytominingxe2x80x9d, is a desirable alternative as a means for recovering metals from soil. Ordinary cultivation methods, however, without adequate preparation and maintenance of soil conditions, does not lead to adequate hyperaccumulation of metals in the plants economically interesting. Additionally, specific methods for recovery of the metals remain to be explored.
Among the soil conditions and cultivation methods most frequently investigated, the relationship between calcium levels and nickel uptake, as well as nickel tolerance, have been frequently reported. While the reports are not uniform, in general, the prior art has reported a negative correlation between calcium concentration and nickel upgrowth. Gabbrielli et al., Atti. Soc. Tosc. Sci. Nat. B38: 143-153 (1981) observed that serpentine soils typically have low levels of calcium. An increase in calcium level was reported to reduce nickel uptake. Similarly, increasing Mg and Ca has been reported to lower nickel tissue concentration in nickel accumulator species endemic to serpentine soils. Gabgrielli et al., Physiol. Plant, 62:540-544 (1984). See also, Vergano et al., The Vegetation of Ultramafic Soil, page 319-322, (1992). Thus, in general, the art teaches that raising calcium levels from the extremely low values normally encountered in serpentine soil to higher levels can be expected to yield a reduction in nickel uptake.
Similarly, a ratio recognized as important in maintaining the health of various plants endemic to serpentine soils is the exchangeable Ca/Mg ratio. Prior art reports set a ratio of about 0.67 recommended as a fertility index. Alexander et al., Soil Sci. 149:138-143 (1990). Typically, exchangeable Ca/Mg ratios in serpentine soils are at much lower values of about 0.2. Thus, the general teaching of the art is that to preserve fertility, a substantial increase in available calcium is required, which can be expected to decrease nickel uptake.
In U.S. patent application Ser. No. 08/470,440, which is incorporated in its entirely herein by reference, a method of phytomining is disclosed which calls for reduction of calcium levels, among other soil treatments. This is consistent with teachings of the prior art. Accordingly, it remains an object of those of skill in the art to develop a reliable system for phytomining of soils rich in nickel, cobalt and the other identified metals, naturally occurring or, otherwise, that will lead to a recovery of these metals at economically acceptable levels.
By screening a wide variety of plants from the Brassicaceae family, the inventors have identified plants in the Alyssum genus which may be hyperaccumulators of nickel and which accumulate valuable amounts of cobalt. By definition, hyperaccumulator plants accumulate over 1000 mg Ni or Co/kg dry weight growing in the soils where they evolved. Because cobalt occurs at about 3-10% of the level of Ni in the listed soils, Ni is the dominant toxic metal which induced evolutionary selection of the Ni hyperaccumulator plants and Co is accumulated to economically useful levels but Ni hyperaccumulation is the dominant economic benefit of the phytomining technology. Evidence suggests members of the section Odontarrhena of the genus Alyssum are likely candidates as nickel hyperaccumulators. The plant may also concentrate, in the above-ground plant tissues, metal from the platinum and palladium families, including Pd, Rh, Ru, Pt, Ir, Os and Re, in significant amounts. Accumulation of nickel in plant tissues in excess of 2.5 percent is practicable.
The metals listed accumulate in biomass by growing nickel hyperaccumulating Alyssum species in the target soils. Some 48 taxa within the section Odontarrhena of the genus Alyssum are known to be hyperaccumulators of nickel. These include the following species already evaluated: A. murale, and A. pintodasilvae (A. serpyllifolium ssp.), A. malacitanum, A. lesbiacum, and A. fallacinum. Other Ni-hyperaccumulating species which may be employed include: A. argenteum, A. bertolonii, A. tenium, A. heldreichii. About 250 other plant taxa have been shown to hyperaccumulate nickel, but many of these do not exceed 10,000 mg Ni/kg d.w., and the majority are of tropical origin.
The identified metal species are accumulated by growing the Alyssum in nickel-rich soil, under specific soil conditions. The conditions include: (1) lowering the soil pH, which increases the phytoavailability of nickel; (2) maintaining moderate levels of Ca in the soil by appropriate treatments and by use of Ca. Mg-rich soil amendments adjusted to maintain Ca levels at levels corresponding to solution values between 0.128 mM and 5.0 mM: (3) using ammonium constraining or ammonium-generating nitrogen fertilizers to improve plant growth and to increase Ni hyperaccumulation due to rhizosphere acidification; and (4) applying chelating agents to the soil to improve nickel uptake by the roots of the hyperaccumulating Alyssum species. Examples of suitable chelating agents include nitrilotriacetic acid (NTA). Other chelating agents commonly used in connection with increasing soil metal mobility for plant uptake include ethylenediaminetetraacetic acid, and ethylene glycol-bis-(xcex2-aminoethylethehr)-N, N-tetraacetic acid. Maintenance of these soil-conditioning factors will improve nickel hyperaccumulation in Alyssum, in excess of a 2.5 percent concentration in above-ground portions of the plant, particularly leaves and stems or shoots, which make for easy cultivation and metal recovery. This is preferable to concentration in the, roots, discussed in Raskin et al., which may be an aid in soil rededication if non-leachable therefrom, but does not offer convenience for phytomining. It is particularly surprising that intermediate values of Ca increase Ni uptake while values of 0.128 mM and below and 5 mM and above decrease Ni uptake. This, combined with exchangeable Ca/Mg ratios of 0.16-0.40, much lower than that recommended in the prior art, further increases Ni tissue concentrations.